1. Field of the Invention
The present invention relates to a rotor with distributed windings used in a rotary electric-machine of an electric plant such as a variable-speed pumped storage generating system, presently under development, a variable-speed fly-wheel generating system, a variable-speed reactive power phase modifying system and a variable-speed frequency converting system which will be developed in future.
2. Description of the Related Art
The development of pumped storage power stations has greatly progressed recently as the demand for electric power increases, and has contributed to keeping a balance between the supply and demand for power. However, the demand for power increases every day, as does the amount of electricity generated by nuclear power plants. Thus, the need to control this power and use it efficiently, for example, at midnight and during holidays is enhanced. To meet this need, a variable-speed pumped storage generating system involving the latest power electronics technology and large-sized rotary machine technology has been put to practical use.
The variable-speed pumped storage generating system creates the following operational advantages unobtainable from conventional power generating systems: the frequency can be controlled to absorb variations in the demand for power in the pumping operation; power generating efficiency is improved; a power system can be stabilized since its input/output control can be executed instantaneously using the rotation energy of a rotor; the voltage of a power system can be maintained and stabilized since reactive power is controlled quickly and widely, etc.
For a generator/motor of the variable-speed pumped storage generating system, a rotating magnetic field, which rotates at a slip speed, has to be formed on the rotor. To do so, a three-phase distributed winding is wound on the rotor, as it is wound on a stator. This rotor has the same structure as that of a rotor of a wound-rotor type induction motor.
The largest amount of power generated from the conventional wound-rotor type induction motor in Japan is 27 MW. The diameter of the rotor of this induction motor is 3.3 m, and the core length thereof is 1.1 m. In contrast, a variable-speed generator/motor capable of generating power of 85 MW and 310 MW have been already manufactured. The rotor of the 85 MW generator/motor has a diameter of about 7 m and a core length of about 2 m, and that of the 310 MW generator motor has a diameter of about 5.5 m and a core length of about 4 m.
While the maximum peripheral speed of the rotor is about 100 m/s in the wound-rotor type induction motor, that of the rotor is 110 to 130 m/s in the variable-speed generator/motor. In the variable-speed generator/motor, the synergism of the increase in generated power and size of rotor winding (the sectional area of which is four to five times as large as that of the conventional winding) causes the centrifugal force exerted upon the rotor winding on the rotor to be ten or more times as much as that of the conventional rotor.
The most important object to be attained in order to manufacture a variable-speed generator/motor involving a rotor which has never been seen before, is to resolve various problems regarding a device for supporting the centrifugal force exerted upon the end portion of the rotor winding of the rotor which has a larger diameter and a higher peripheral speed than that of the wound-rotor type induction motor.
More specifically, by supporting the centrifugal force using an efficient supporting device, the stress generated at the winding end portion has to be leveled and reduced to maintain a good insulation state in the winding for a long time. As is already known, in the conventional wound-rotor type induction motor, a nonmagnetic steel wire or the like is wound around the winding at its end portion to support the centrifugal force generated thereon, and the rotor winding end portion is thus supported by the tension caused by the winding of the nonmagnetic steel wire.
Various other objects, features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood from the following detailed description when considered in connection with the accompanying drawings in which like reference characters designate like or corresponding parts throughout the several views and wherein:
FIG. 1 is a view showing the appearance of a rotor of the above-described wound-rotor type induction motor, and FIG. 2 is an enlarged detailed sectional view of rotor winding end portions.
Referring to FIG. 1, a cylindrical rotor 1 includes a rotor core 1A, a rotating shaft 6, and a nonmagnetic steel binding wire 26. As illustrated in FIG. 2, the rotor 1 also includes a supporting ring 27, spacers 28A and 28B, a stator core 29, and a stator winding 30. A space G.sub.L is formed between the rotor core 1A and stator core 29. As shown in FIG. 2, rotor winding end portions 2A and 2B are fixed in the rotor core 1A by a wedge (not shown). The binding wire 26 is wound around the peripheries of the winding end portions 2A and 2B projected from the end of the rotor core 1A. The centrifugal force exerted upon the winding end portions 2A and 2B during the rotation of the rotor is therefore supported by the tension of the binding wire 26.
If the above mechanism is applied to a variable-speed generator/motor which is larger in size and higher in peripheral speed than the wound-rotor type induction motor, the following drawback will arise. Since, as described above, the centrifugal force acting on the rotor winding is ten or more times as much as that in the conventional case, the number of times the binding wire 26 is wound needs to be increased. However, the rotor is made to pass through the ring-shaped stator core 29 when the generator/motor is installed, so that the outside diameter of each rotor winding end portion has to be smaller than the inside diameter of the stator core 29, which places restrictions on the number of windings of the binding wire 26. As the rotor is increased in power and speed, the elastic deformation of the binding wire 26 becomes greater, and the winding end portions are displaced widely and thus cannot be supported sufficiently.
As shown in FIG. 2, the winding end portions 2A and 2B are covered with the binding wire 26 and supporting ring 27 and, in this case, the current flowing through the rotor windings of the wound-rotor type induction motor is small, and the amount of heat generated therefrom is also, in accordance, small. For this reason, there occurs no problem in cooling even though the winding end portions 2A and 2B are covered with a supporting structure.
Since, however, the variable-speed generator/motor generates high power, a large amount of current flows through the rotor windings, and a large amount of heat is generated. If the heat is prevented from radiating by a supporting structure, there occurs problems relating to overheating and the deterioration of insulation, thus requiring a supporting device with good ventilation which does not prevent heat radiation.
The foregoing drawbacks will occur if the technique of supporting the rotor winding end portions is applied to a large-sized, high-power, variable-speed generator/motor. Even though the technique of the conventional device for supporting the winding end portions, which is employed in a small-sized, low-power, induction motor, is applied to the variable-speed generator/motor, a desired function is not fulfilled, and the reliability of the supporting device is greatly reduced. In view of this, the need to develop an excellent supporting device has increased.
Furthermore, the conventional supporting device has the following drawback regarding the assembly and installation of the variable-speed generator/motor. When the aforementioned supporting device, on which the binding wire is wound, is applied to the variable-speed generator/motor, a winding device including a rotating unit and a tension generating unit is required to wind the binding wire during assembly of the rotor.
FIGS. 3A and 3B are views showing a method for winding the binding wire on the rotor of the wound-rotor type induction motor. As shown in FIGS. 3A and 3B, the rotor 1 is supported by a temporary shaft 31 received by temporary bearings 32A and 32B. When the binding wire 26 is wound around the rotor 1, the rotor is rotated through a drive belt 33A driven by a rotating unit 33, and a tension generating unit 34 applies appropriate tension to the binding wire. An operator works on a workbench 35.
As is apparent from FIGS. 3A and 3B, the area of the floor necessary for the winding operation is several times as large as the projected area of the rotor. In the wound-rotor type induction motor, however, the area of the floor is not important since the winding operation is performed in its dedicated space of a plant.
By contrast, since the rotor of the variable-speed generator/motor is large in size, its component parts are transported from a factory to a power station, and assembled into the rotor in an assembly room of the power station. If, therefore, the conventional supporting device is adopted, the floor of the assembly room has to be enlarged in the power station, in view of the winding operation.
The generator/motor is installed in an underground hydroelectric power station in relation to a water power site. Therefore, a large assembly room occupied in the power station increases the cost of excavation.
The time required for installing a generator/motor in a power station, will now be described. In general, a high revolution speed (400 rpm or more) rotary electric-machine generating electric power of 300 MW, which is one of the conventional constant-speed generator/motors, can be assembled in about nine months, whereas a rotor of the machine can be assembled in about a third of that time. Since the assembly of the rotor is performed, together with the assembly of a stator and the like, the time needed to assemble the rotor exercises no influence on the total time required to assemble the machine.
In a variable-speed generator/motor generating an electric power of 300 MW, a rotor having a device for supporting a winding end portion on which a binding wire is wound, is assembled in about nine months. This period is as long as the total time needed to assemble the constant-speed generator/motor and is mainly due to the time necessary for winding the binding wire on the winding end portion of the rotor. For this reason, the total assembly time required for the variable-speed generator/motor amounts to about one year, the construction costs are increased, and starting date of operation is often delayed, which can involve a large financial loss.
The time required for replacing a long-used rotor winding (distributed winding) with a new one will now be described. This period amounts to about six months and is six or more times as long as the time required for exchanging the rotor windings (field windings) of the constant-speed generator/motor. Therefore, it is economical risky to stop a power plant capable of generating high power of 300 MW for as long a period as six months.
Assume that unexpected damage occurs on a single rotor winding (distributed winding). Even in this case, it would take about four months to repair it, and a power plant has to be stopped accordingly.
Furthermore, if damage is caused to a rotor winding after a rotor is installed, when it is replaced with a new one, an operation of winding a binding wire on the new rotor winding is needed. It is thus necessary to always equip a power plant with a winding device including a rotating unit, resulting in an increase in installation costs and the space required for accommodation. As described above, if a supporting device, conventionally applied to a small-sized power station, is applied to a large-sized one, it adversely affects the assembly and installation of a rotor directly and indirectly, thereby increasing construction costs.
Recently a new type of supporting device has been taken into consideration in order to eliminate the aforementioned drawbacks, and is proposed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2-219430.
According to the publication, a cylindrical supporting ring is disposed around a rotor winding end portion, and they are coupled to each other by means of a number of radial-direction coupling members such as U-shaped bolts and stud bolts, thereby supporting the centrifugal force acting on the rotor winding end portion. Consequently, in this publication, some of the drawbacks of the prior art technique are overcome by reducing the area of the floor of an assembly room occupied in the power station, shortening the time required for assembling a rotor and omitting a device used exclusively for assembling the rotor, etc.
However, in order to apply the Japanese prior art to a large-sized, high-power, high-speed rotary electric-machine, a number of problems with the reliability of the support device, the economics of producing component parts of the supporting device and the like have to be eliminated. Therefore, a highly reliable and economical support device capable of being applied to a high-speed rotary electric-machine is needed. In view of these problems, the support device disclosed in the Japanese publication will now be described in detail.
In the supporting device of this publication, a number of ventilation holes and bolt penetration holes are formed in a cylindrical supporting ring in the radial directions of the supporting ring. These holes are a great hindrance to the design and manufacture of the supporting ring.
The first problem being as follows: Since the centrifugal force and external force are exerted on the supporting ring, stress is concentrated around the formed holes, and the resistance, of the supporting ring, to fatigue is considerably decreased. Therefore, reliability in terms of strength is very low.
The second problem is as follows: Since the cylindrical supporting ring is large, a great number of holes are formed on the surface of the supporting ring (e.g., about 5000 holes in the rotary electric-machine generating power of 300 MW), and the unit time for forming the holes is lengthened, thereby considerably increasing the time and cost involved in forming the holes.
The higher the revolution speed of the machine and the larger the amount of power generated therefrom, the greater the influence of the first and second hindrances. It is thus necessary to develop a reliable, economical supporting ring which is resistant to fatigue and can easily be manufactured at low cost even when it is applied to a high-speed, high-power, rotary electric-machine.
The problem of ventilation holes will now be described. As described above, a number of stud bolts or U-shaped bolts are required in order to apply the conventional supporting ring to a high-speed, high-power machine. Bolt penetration holes are formed in sequence in the supporting ring in the circumferential direction and in the axial direction perpendicular thereto. Since the supporting ring has no spaces for ventilation holes because of a great number of penetration holes, no fresh air can be circulated in a rotor to cool the rotor winding end portions. Taking any other measures to ventilate the rotor would complicate the constitution of the rotor. It is thus essential to develop a supporting ring capable of sufficient ventilation for cooling the rotor winding end portions despite a number of stud bolts or U-shaped bolts even when it is applied to a high-power, high-speed rotary electric-machine.
The contact pressure of an upper rotor winding 42A and an insulating block 426, which is caused by the centrifugal force between them, will now be considered, with reference to FIGS. 4A, 4B, and 5. Assuming that the width of the insulating block 426 is w.sub.b and that of the upper rotor winding 42A is w.sub.c, their contact area is represented by w.sub.b .times.w.sub.c. In FIG. 4A, numeral denotes a supporting ring, numeral 424 denotes a U-shaped ring and numeral 424 denotes a nut.
The centrifugal force exerted between the upper and lower rotor windings 42A and 42B is supported by the contact surface of the upper rotor winding 42A and insulating block 426. Thus the contact pressure, which is inversely proportional to the contact area, acts on an insulating portion of the upper rotor winding 42A.
Moreover, the contact pressure has to be prevented from exceeding a predetermined value (50 MPa or less). In order to maintain the reliability of the insulating function of the winding end portions and lengthen the lifetime thereof, it is preferable that the contact pressure should be as low as possible. It is thus desirable to arrange the insulating block 426 and U-shaped bolt 425A on the upper rotor winding 42A so as to obtain a large contact area of w.sub.b .times.w.sub.c. Since, however, the U-shaped bolt 425A and insulating block 426 cross the upper rotor winding 42A in FIGS. 4A, 4B, and 5, the contact area of the given widths w.sub.b and w.sub.c is the smallest.
Particularly in a high-power, high-speed rotary electric-machine, the centrifugal force of the upper and lower rotor windings 42A and 42B is increased, as is the contact pressure thereof. Therefore, the U-shaped bolt 425A and insulating block 426 need to be arranged effectively so that the contact area of the given widths w.sub.b and w.sub.c can be enlarged.
What is more important, the bending stress of winding end portions caused by the centrifugal force has to be supported so as to have a value safe enough for the bending strength of the rotor windings, in order to maintain a good insulating function in the winding end portions for a long time. In other words, the pitch between U-shaped bolts 425A shown in, e.g., FIG. 5 has to be restricted in order to set the bending stress of the winding end portions to a proper value of 30 MPa or less in accordance with a material of the winding end portions.
The pitch between the rotor winding 42A supported by the U-shaped bolts 425A and rotor core 41, as shown in FIG. 5, has to be set at an appropriate value since the pitch influences the bending stress of the winding end portions greatly, as does the pitch between the U-shaped bolts of the same rotor winding. The difference between the former and latter pitches also has to be set at an appropriate value.
As the revolution speed of a rotary electric-machine is increased and the amount of power generated therefrom is also increased, the above pitches need to be narrowed and leveled. However, in a rotary electric-machine whose supporting ring 422 is fixed directly to a presser plate, the pitch between the rotor core 41 and U-shaped bolt 425A cannot be set to a small value because of structure restriction. It is thus difficult to cause the bending stress of the winding end portions at the end of the rotor core to fall within a tolerance.